Conventionally, a disc brake is utilized as a brake device of a passenger car, and a disc brake pad made by fixing the friction material on a base member made of metal such as steel is utilized as a friction member of the disc brake.
The friction material is classified into a semi-metallic friction material containing, as a fiber base, 30 weight % or more but less than 60 weight % of a steel fiber relative to the total amount of the friction material composition, a low steel friction material containing a steel fiber in a part of the fiber base as well as less than 30 weight % of the steel fiber relative to the total amount of the friction material composition, and the NAO friction material containing no steel-based fiber such as the steel fiber and a stainless steel fiber.
The friction material generating less braking noise is demanded of late years, it is a recent trend to use the disc brake pad that uses the NAO friction material that does not contain the steel-based fiber but mainly contains such as a binder, a fiber base material, a lubricant, a titanate, an inorganic friction modifier, an organic friction modifier, pH adjuster, and a filler.
For the NAO friction material used for the brake pad, in order to secure the fade resistance and the wear resistance, 5-20 weight % of a copper component such as fibers and/or particles of copper and/or copper alloy relative to the total amount of the friction material composition, is added as a necessary component.
However, recently, the above-described friction material, when braking, discharges the copper as abrasion powder, and it is suggested that the discharged copper flows in a river, lake, and/or ocean and then the copper possibly contaminates an area around the discharged copper.
Because of these background, for example, California State (CA) and Washington State (WA) of the United States of America passed a bill to prohibit the sales of the friction member using the friction material containing 5 weight % or more of the copper component relative to the total friction material composition and an act of assembling the subject friction material in a new car from the year of 2021, and the sales of the friction member using the friction material containing 0.5 weight % or more or the copper component relative to the total amount of the friction material composition and an act of assembling the subject friction material in a new car from the year of 2025.
Then, as this type of regulations is expected to be spread out in the world from now on, the elimination of the copper component contained in the NAO friction material is urgently needed, and a primary issue is to improve the fade resistance and wear resistance that could be reduced due to the elimination of the copper component contained in the NAO friction material.
The Patent Document 1 discloses the friction material, which is manufactured by forming the friction material composition containing 0.5-50 weight % of the metallic tin or tin alloy relative to the total amount of the friction material composition and 0.001-4.999 weight % of the copper relative to the total amount of the friction material composition.
The Patent Document 2 discloses the nonasbestos friction material composition containing a binder, an organic filler, an inorganic filler, and a fiber base material where the amount of the copper contained in the friction material composition as the copper element is 5 mass % or less and the metallic fiber other than the copper and copper alloy contained therein is 0.5 mass % or less while the nonasbestos friction material composition contains the mica and the graphite with the particle diameter of 90 μm or less as the inorganic filler but does not contain graphite with the particle diameter of over 90 μm as the inorganic filler and the friction material, which is manufactured by forming the nonasbestos friction material composition.
However, the friction materials disclosed in the Patent Document 1 and Patent Document 2 are satisfying laws relating to the required amount of the copper component contained therein but cannot be said to sufficiently prove the demanded performance in the fade resistance and the wear resistance.
The Patent Document 3 discloses the friction material that contains the fiber base material, the resin binder, and the filler, in which as the filler, a natural scaly graphite is flaked and granulated to obtain a granulated flake graphite with the average particle diameter of 100-1000 μm and the bulk density of 0.10-0.20 g/cm3.
The granulated flake graphite is formed by cleaving the natural scaly graphite to obtain a pre-mixed material, mixing the pre-mixed material with coal tar pitch or bulk mesophase pitch or graphitized binder such as phenolic resin and polyimide resin to obtain a pre-kneaded material, kneading the pre-kneaded material to obtain a pre-granulated material, granulating the pre-granulated material to obtain a pre-baked material, and then baking and carbonizing the pre-baked material. Also, the granulated flake graphite is formed by crushing the above baked and carbonized material. Here, the particle diameter of the granulated flake graphite is relatively large, and the bulk density is small so that the porosity of the friction material can be appropriately increased.
The granulated flake graphite is such that the entire particle is coated with the graphitized binder and the thermal conductivity and conductivity that the flake graphite naturally can be hindered, and therefore no other advantage than the increase of the porosity can be expected.
In addition, eliminating the copper component contained in the friction material clearly has various problems in the manufacturing process which became obvious.
The disc brake pad is manufactured through a mixing step of uniformly mixing the predetermined amount of the friction material composition by a mixer, a heat press forming step of heat press forming the obtained raw friction material mixture and a back plate, which is separately cleaned, surface treated, and adhesive applied, to be positioned in a heat forming die with the raw friction material mixture superposed thereon, a heat treatment step of heating the obtained molded article to complete the curing reaction of the binder, a coating step of coating the obtained article, a baking step of baking the coated article, and a grinding step of forming a friction surface by a grinder. Also, after the heat press forming step, it is possible to use a heat treatment step functioning both coating and baking steps and then the grinding step comes thereafter.
For the coating step, because of a better coat film, easier to collect the coating material, and less environmental impact because of the volatility organic solvent than the spray coating that uses liquid coating material such as organic coating solvent, an electrostatic powder coating that uses the powder coating as in the Patent Document 4 is generally employed.
Coating has been applied on an outer surface of the metallic back plate to prevent rusting; however, recently to increase the value of the disc brake pad, coating is applied on a peripheral side surface of the friction material except the friction surface as well as an outer surface of the back plate.
When the friction material includes the metallic material, the friction material has conductivity and therefore just like the metallic back plate, the friction material may be coated with the electrostatic powder coating. However, by eliminating the copper component, the conductivity of the friction material significantly decreases and the adhesiveness of the powder coating is weaken due to the static electricity, and therefore the coating on the friction material side surface becomes difficult.